1. Field of the Invention
The invention relates to a device for monitoring and compensating for change in the flight time of the print hammers of impact printers.
2. Description of the Prior Art
In impact printing, the printing process is carried out by the movement and impact of a print hammer against a passing print type carrier, which subsequently strikes an impact platen. Printing type carriers commonly used are steel belts with etched types, belts with dot elements and daisy wheels.
The time between the moment of firing of the print hammer and the moment at which the print type carrier strikes the impact platen is commonly referred to as the flight time. In high speed line printers, the timing sequence for firing the print hammers must be very precise in order to ensure that the desired print type is aligned in its proper position when the hammer impacts against it. This precision requires that the flight times of the print hammers be known and the moments of impact be relatively constant. However, the flight times of print hammers in high speed line printers are gradually and continually changing as the machine is in use. Possible reasons are wear of the print hammer surfaces, change of the flight distance to be covered, change in the mechanical friction of the hammer firing mechanism, variations of the electrical voltage within the printer electronics, temperature variations, etc. Changes of the flight time have a direct and significant influence on the quality of the printing.
In printers having individual print hammers at each printing position, the frequency of use of each individual print hammer affects its flight time independent of the other print hammers. Therefore, it is necessary to periodically adjust the flight times of all the print hammers so that flight time remains a constant. Typically this is done by manual adjustment of the hammer flight distances. This manual adjustment is very costly, very time consuming, and does not prevent a deterioration of the print quality in the interval between two manual adjustments.
It is desirable to replace the manual adjustment procedure with an automatic adjustment procedure in which differences in flight times would be compensated for by shifting the moments of firing of each individual print hammer. By shifting these firing times, the impact times of each print hammer will be precisely maintained in the timing pattern of the printing sequence. In order to be able to automatically adjust the firing time of each print hammer, the precise moment of impact of the print type carrier on the impact platen must be determined. Prior art devices for monitoring the flight times of print hammers of impact printers primarily utilize two approaches.
In the first approach, a test line is periodically printed, the flight times are measured, and flight time differences of the individual print hammers are electronically compensated for. A considerable disadvantage of this method is that the separate test line is actually printed, which is not particularly desirable. Disengagement of the ribbon is not a practical solution, as it is inconvenient and typically not effective since carbon paper is frequently used.
The other principal approach involves the measurement of the flight time of each individual print hammer during standard printer operation. A disadvantage of this method is that the flight times of print hammers in print positions that are not used in a particular print line cannot be adjusted. Furthermore, the flight times of the individual print hammers overlap so that their impact times cannot easily be associated with their particular print position, unless a separate transducer or sensor for the moment of impact is provided for each individual print hammer. This method can be quite expensive and complex, and frequently causes interruptions of the other printer functions.
An automatic device for monitoring and compensating for change in the flight times of each and every individual print hammer that overcomes these shortcomings is desirable.